Continuous intrathecal infusion of concentrated morphine is widely used in pain therapy. Surprisingly, until recently there has been no study of the safety of such infusions. We investigated the effects of 28-day intrathecal morphine infusion in a canine model. Unexpectedly, at high morphine concentrations (as used in humans), we noted an aseptic mass of inflammatory cells (granuloma) arising from the dura-arachnoid, not the parenchyma, proximal to the catheter tip. Granulomas were not seen with vehicle or a variety of non-opioid agents. The alpha2 adrenergic agonist clonidine suppressed the granuloma. These observations lead to four hypotheses. 1. Granuloma induction by morphine is proportional to local concentration in cerebrospinal fluid and not simply total dose. 2: Effect is mediated by an opioid agonist action and is not limited to morphine. 3. The granuloma results from a local degranulation of dural mast cells leading to movement of inflammatory cells from the dural vessels. Accordingly, granuloma-inducing potency will be proportional to the ability to degranulate dural mast cells in ex vivo dural preparations. 4. Granuloma-inducing effects and dural mast cell activation are suppressed by local alpha2 receptor agonists and by a mast cell stabilizer. We will address these hypotheses using the canine model to examine the effects of continuous intrathecal infusion of equipotent doses of mu opioid agonists (morphine, morphine-6-glucuronide, L-methadone, hydromorphone, fentanyl or DAMGO) or equimolar concentrations of inactive opioid molecules (naloxone, morphine-3-glucronidc, D-methadone). In vivo treatment with a mast cell stabilizer, nedocromil sodium, will be examined for its effect on granuloma formation. In parallel studies, kinetics studies will permit comparisons based on measured CSF concentrations. Interaction between morphine and alpha2 agonists (clonidine, dexmedetomidine) will be studied by co-delivery. Granuloma formation and local mast cell degranulation and cytokines will be assessed histochemically and by CSF analysis. In summary, our initial work, provides the first definitive preclinieal data defining the effect, the attenuation by clonidine, and a novel mechanistic hypothesis for drug-induced degranulation of dural mast cells which suggests a novel method for the ex vivo screening of new agents. These studies are significant: 1) increasing incidence of reports of morphine-granulomas emphasize it is not rare; 2) our investigation of other opioids provide the first time assessment of the spinal safety of agents which are now in wide clinical use; and 3) this issue impacts on all agents targeted for intrathecal delivery. Accordingly, data obtained here regarding the role of local CSF concentration, the safety of non-morphine agents and the potential ameliorating effects of adjuvant agents all provide novel information to refine the utility of this important therapeutic regime.